It is known to connect hubs in a closed system to enable a user to be connected to any unit in the stack and to enable the stack to be managed as a single logical entity. It is necessary to provide some means of arbitration between requests by the units for access to the ring. It is known to provide a ring communication system in which a unit having data to transmit monitors frames circulating on the ring and if a token bit in a frame header indicates that the token is `free` the unit may transmit and also set the token bit so that other units may not transmit while the unit that has set the token still has `possession` of the ring. A system of this nature can include a priority indication which prevents the issue of a free token. This inhibition is chiefly intended to allow synchronous data packets to take precedence over asynchronous data packets, as described in U.S. Pat. No. 4,482,999.